The present invention relates to packaging materials of a type employing flexible, polymeric films. More specifically, the invention pertains to multilayer packaging films used in applications requiring a high degree of dimensional stability, i.e., both low shrinkage and low stretch, at elevated temperatures, and also a relatively low oxygen transmission rate.
Packaging applications requiring dimensionally stable films at high temperatures, e.g., up to about 120xc2x0 C. and sometimes as high as about 130xc2x0 C., include vertical form-fill-seal (VFFS) packaging for xe2x80x9chot fillxe2x80x9d products, such as soups, sauces, jellies, beverages, and other liquified foods, which are normally maintained at about 80xc2x0 C. to about 100xc2x0 C. during packaging. As is well known, in VFFS packaging, a flowable product is introduced through a central, vertical fill tube and into a formed tubular film having been heat-sealed transversely at its lower end and longitudinally. After being filled, the package, in the form of a pouch, is completed by transversely heat-sealing the upper end of the tubular segment, and severing the pouch from the tubular film above it, usually by applying sufficient heat to melt through the tube above the newly formed upper heat-seal. If the film from which the package is made does not have sufficient dimensional stability, the package becomes distorted both from the heated product and from the heat-sealing. Not only does package distortion ruin the aesthetic qualities of the package, e.g., by distorting any printed information or other labeling on the package, but it can cause the package to become mis-aligned in the packaging machinery, often resulting in ruined packages and costly downtime in production as mis-aligned packages become wedged between pieces of the machinery or when heat-sealing/severing equipment inadvertently contacts and melts through the walls of the package instead of sealing and severing at the periphery of the package as intended.
Similar considerations apply in VFFS and horizontal form/fill/seal (HFFS) packaging of flowable particulate products, e.g., shredded cheese, frozen chicken wings and nuggets, etc. Although such products are generally not filled while in a heated state, transverse and longitudinal heat-sealing and heat-severing alone are sufficient to cause package distortion, thereby making a film which is dimensionally stable at elevated temperatures highly desirable for such packaging applications.
Another packaging application for which high-temperature dimensional film stability would be desirable is when films are used as lidding materials for: flexible packages such as thermoformed pockets for, e.g., hot dogs, lunch meats, etc.; semi-rigid vacuum and/or gas-flushed packages for meat and poultry contained in a foam or other semi-rigid type tray; and rigid packages, e.g., for yogurt, custard and other dairy products contained in a rigid cup-like container. When lidding films are applied to such packages, heat is generally used to seal the film to the thermoformed container, tray, or cup in which the product is contained. Without sufficient dimensional stability, the lidding films can either stretch or shrink during the lidding process, resulting in incompletely sealed packages and distorted printed images on the films.
A further process necessitating dimensional stability at elevated temperatures is printing. Maintenance of color-to-color registration on the printing press is important, as is overall consistency of the xe2x80x9crepeat lengthxe2x80x9d of each printed image. Drying tunnel temperatures commonly reach temperatures of 200 degrees F. (93 degrees C.). It is therefore preferred that the film have sufficient resistance to stretching, necking and other types of deformation at these temperatures so that registration is not lost, and that the repeat length of the images are consistently maintained on downstream packaging equipment, where it may again face elevated temperatures as noted above.
Films that are dimensionally stable at high temperatures would generally tend to be relatively stiff at room temperatures. This attribute is highly desirable when the film is made into a stand-up pouch for, e.g., soups, sauces, beverages, and particulates, when it is thermoformed into a pocket and lidded, and when it is used as a lidding film. Thus, not only would a film having high-temperature dimensional stability be able to withstand the rigors of the packaging process without distortion, but the resultant package would be stiff, which is advantageous in certain packaging applications such as those listed immediately above.
Another requirement of films used in many of the aforementioned packaging applications is a low transmission rate of oxygen in order to preserve and extend the shelf life of packaged food products. For many food products, the oxygen transmission rate (OTR) must be on the order of 40 cc/m2 per 24 hours at 1 atmosphere or less.
In order to achieve the above properties, many conventional packaging films used for such applications have been laminates, i.e., two or more film components that are adhesively bonded together, e.g., biaxially-oriented and heat-set polypropylene, polyester, or polyamide films that are adhesively laminated to a heat-sealable film where one of the laminated film components contains a low OTR material such as polyvinylidene chloride. However, adhesive lamination is expensive, due to the relatively high cost of the adhesives and the extra production steps required to produce the laminate, and the reliability of such adhesives is sometimes suspect, e.g., solvents from printing inks can reduce the bond-strength of the adhesives, leading to delamination. Further, certain types of adhesives contain migratable components that can migrate through films and contact the packaged food products.
Instead of using a laminate, it would be preferred to use a film that is fully coextruded, i.e., formed by extruding two or more polymeric materials through a single die with two or more orifices arranged so that the extrudates merge and weld together into a multilayer structure.
A proposed coextruded film having high-temperature dimensional stability and a low OTR includes a core layer of ethylene/vinyl alcohol copolymer (EVOH) bonded on both surfaces to layers comprising amorphous nylon, which may or may not be blended with a crystalline polyamide such as polyamide 12, 612, 6/66, etc. While amorphous polyamide is advantageous in that it provides relatively high modulus at high temperatures, thereby imparting high-temperature dimensional stability to a film in which it is incorporated, amorphous polyamide exhibits poor adhesion to EVOH, resulting in a film that will too easily delaminate. Blending crystalline polyamide with amorphous polyamide improves the bond strength to EVOH, but at the cost of greatly decreasing the modulus of the amorphous polyamide layers, and therefore of the entire film, at high temperatures.
Accordingly, there is a need in the art for a fully coextruded, multilayer film having a combination of high-temperature dimensional stability, low OTR, and sufficient inter-laminar bond-strength to be useful for commercial packaging applications.
That need is met by the present invention which provides a multilayer film, comprising, in the following order:
a. a first layer consisting essentially of amorphous polyamide;
b. a second layer adhered to a surface of the first layer; and
c. a third layer adhered to a surface of the second layer, the third layer comprising at least one member selected from ethylene/vinyl alcohol copolymer, polyamide MXD6, polyamide MXD6/MXDI, polyvinylidene chloride, and polyacrylonitrile. Such materials provide the film with a low oxygen transmission rate, i.e., less than or equal to 30 cc of oxygen per square meter of film per 24 hour period at 1 atmosphere and at a temperature of 73xc2x0 F. (at 0% relative humidity).
Advantageously, the second layer is adhered to each of the first and third layers at a bond strength of at least 0.5 lbf/inch. In addition, the film has a storage modulus of greater than 30,000 pounds/in2 at 120xc2x0 C. for excellent dimensional stability at elevated temperatures.